Many devices have been reported for concentrating incident electromagnetic radiation onto a receiving element such as a photovoltaic cell. For example, in "Design of Optimal and Ideal 2-D Concentrators with the Collector Immersed in a Dielectric Tube," by Juan C. Minano, Jose M. Ruiz, and Antonio Luque, Appl. Opt. 22, 3960 (1983) the authors observe that the geometrical concentration of a compound parabolic concentrator can be increased by a factor of the index of refraction when a dielectric transparent medium is used to fill the concentrator. Additionally, in U.S. Pat. No. 4,114,592 for "Cylindrical Radiant Energy Direction Device With Refractive Medium," issued to Roland Winston on Sept. 19, 1978, the inventor describes the use of a radiant energy transmission device having opposing reflective sides operable in an energy concentrative mode whereby energy incident on an entrance area is directed to and concentrated on an exit area having a smaller dimension than the entrance area. All devices disclosed therein have reflective walls. Of particular relevance to the subject invention is discussion of FIG. 2 thereof in Column 4, lines 50-68 wherein Winston describes the use of nonhomogeneous optical materials having a gradient index of refraction to bend the incoming light rays and consequently shorten the overall length of the device. The gradient best suited to achieve this purpose is described as having the greater index values along the axis of the generally cylindrical device with the values therefor falling away from this axis. This is known as a purely radial gradient. There is no teaching relating to a longitudinal variation of optical density in addition to the radial variation. Also of relevance to the subject invention is FIG. 3 thereof which teaches the combination of a nonhomogeneous refractive media and a reflective wall. Two media are shown, but an infinite number are possible. The media increase in optical density as a function of the radius of the device. The stated purpose for utilizing such a distribution is to reduce the cost of the overall device; that is, the innermost region might be filled with water. See, e.g., Col. 5, lines 64-68 and Col. 6, lines 1-34.
In. U.S. Pat. No. 4,240,692 for "Energy Transmission," issued to Roland Winston on Dec. 23, 1980, the inventor discloses a radiant energy transmitting device operative selectively in a concentrative and emissive modes. Unlike Winston's '592 patent, described hereinabove, where mirrored reflective boundary surfaces are employed, transmitting and guiding surfaces are formed at the interface of media of differing indices of refraction.
Radial refractive optical gradients have been generated in samples of plastic and glass. In U.S. Pat. No. 3,718,383 for "Plastic Optical Element Having Refractive Index Gradient," issued to Robert S. Moore on Feb. 27, 1973, the inventor describes the diffusion of a diluent into a shaped polymeric matrix to form a continuous gradient in refractive index in a direction perpendicular to the optical axis thereof. The diluent and the polymeric material have different refractive indices. In cylindrical samples, an angularly symmetric, radial gradient of refractive index substantially proportional to the radial distance perpendicular to the optical axis may be formed by diffusion of a diluent having lower index of refraction than the plastic matrix material into the matrix from the central core thereof. Similarly, for positive lenses, where the refractive index must decrease in the outward radial direction, inward diffusion of a diluent external to a plastic rod is required.
In U.S. Pat. No. 3,859,103 for "Optical Glass Body Having A Refractive Index Gradient," issued to Mitsugi Yoshiyagawa on Jan. 7, 1975, the inventor describes the production of a continuously decreasing index of refraction from the central axis of a glass object to the peripheral surface thereof as a result of the substitution of thallium ions contained in the glass by external alkali metal ions. Glass containing Tl.sub.2 O was chosen since the thallium ions give the glass a high refractive index. The process for achieving the required substitution of ions is to bring the glass article into contact with a chosen molten salt for a period of time sufficient for significant diffusion to take place. A distribution of the refractive indices according to the relationship N=N.sub.O (1-ar.sup.2) was generated in a glass rod, where r is the distance from the center in the radial direction, a is a positive constant, and N.sub.O is the refractive index at the center of a cross section of the glass body perpendicular to the central axis thereof.
In U.S. Pat. No. 4,053,204, "Optical Fiber Having Reduced Dispersion," issued to Stewart E. Miller on Oct. 11, 1977, and in U.S. Pat. No. 4,076,380, "Graded Index Optical Fiber," issued to Frank Vincent DiMarcello and John Charles Williams on Feb. 28, 1978, the inventors disclose graded refractive index optical fibers having radial gradients in repetitively varying discrete longitudinal zones for improving the dispersion characteristics of light pulses traveling therethrough. In the former patent, the zones are achieved by varying the thickness of each layer of constant optical index material, while in the latter patent, layers of different index of refraction are disposed in a helical pattern along the length of the fiber. Chemical vapor deposition techniques are used to form the layers in both devices.
In U.S. Pat. 4,696,552, "Projection Device with Refractive Index Distribution Type Lens", issued to Jun Hattori and Shigeyuki Suda on Sept. 29, 1987, the inventors disclose a projection device having an illuminating system for illuminating an object, and an index distribution type lens for projecting the image of the object. The lens has a refractive index distribution substantially proportional to the square of the distance from the optic axis in a cross-section perpendicular to the optic axis and a refractive index distribution monotonously varying in the direction of the optic axis. The lens is characterized by dimensions of about 18 mm in length and 0.5 mm in diameter (perpendicular to the optic axis) and a change in refractive index of less than 0.05.
For the purpose of the present specification, we define the term "optical axis" to mean an imaginary straight line which extends internally through the refractive material of the subject invention and which passes through both the entrance and exit surfaces of this material which are adapted for the passage of light. Although there may be more than one optical axis for a chosen embodiment of the invention, in general, the optical axis will be uniquely defined by the geometrical symmetry of the material. In either event, changes in the index of refraction of the refractive material will be defined relative to the optical axis. Also for the purpose of the present specification, we define the term "bidirectional gradient" to refer to a gradient in the index of refraction that occurs along each of two directions, usually mutually orthogonal. Finally, "light" is defined as that electromagnetic radiation in the frequency spectrum ranging from infra-red through visible to ultraviolet.
Notably absent from the patent literature and from the science and engineering literature is a description of transmitting light concentrating and/or directing devices having bidirectionally varying indices of refraction or devices having indices of refraction varying in three dimensions having substantial thickness in the direction of variation of refractive index. Additionally, monotonically varying distributions of optical densities with significant change in index of refraction and over significant dimension in the axial direction have not been described. While Hattori et al, supra, disclose lenses having bi-directional gradient varying indices of refraction, such lenses have no substantial thickness, as the term is used herein, and no significant change in index of refraction, as the term is used herein.
Accordingly, it is an object of the present invention to provide light directing devices having a macro-gradient in the index of refraction, that is, at least about 0.1, and having large geometries, that is at least about 5 mm in the direction perpendicular to the optical axis.
Another object of our invention is to provide non-tracking transmissive optical light concentration and collection devices having greater gain than existing devices.
Yet another object of our invention is to provide transmissive optical systems which are smaller and lighter than existing devices.
Still another object of the present invention is to provide a non-tracking transmissive solar energy collector having a broad acceptance angle.
Another object of our invention is to provide a transmissive light image reducer or enlarger.
Yet another object of the present invention is to provide a process for the fabrication of monolithic glass articles having a significant bidirectional gradient in index of refraction and for the fabrication of glass articles having a varying index of refraction in three dimensions.
Another object of our invention is to generate similar transmission characteristics in a single, integral lens to those provided by at least two individual lenses cooperating as a compound lens.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.